Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of making lithographic printing plates the demand for dry methods to prepare printing plates is ever increasing. Therefore the preparation of such printing plates via non-impact printing (e.g. via ink-jet printing, ionography, magnetography and especially electro(stato)graphy) has received and still receives great interest.
Generally, three different types of lithographic printing plates prepared by electro(stato)graphy have evolved.
One type of printing plate is produced by the following steps:
(i) uniformly electrostatically charging a photoconductive layer, such as a coating of zinc oxide photoconductive pigment dispersed in a resin binder, by means of a corona-discharge, PA0 (ii) image-wise discharging said photoconductive layer by exposing it to electromagnetic radiation to which it is sensitive, PA0 (iii) applying electrostatically charged oleophilic toner particles to develop the resulting electrostatic charge pattern either by positive or reversal development and PA0 (iv) fixing the toner to the photoconductive layer. Fixing is usually accomplished by the use of heat which causes the toner resin powder to coalesce and adhere to the photoconductive layer. PA0 i) providing an image receiving element comprising a support and an image receiving layer wherein said image receiving layer has a kinetic friction coefficient (.mu..sub.k) against rubber, with Shore Hardness 70, .gtoreq.0.85, and PA0 ii) image-wise depositing dry toner particles, either via an electrostatic latent image or via Direct Electrostatic Printing, on said image receiving layer, thus creating an image on a background, said image and said background having a different affinity for lithographic printing ink. PA0 i) creating an electrostatic latent image on a latent image bearing element and developing it with dry toner particles thus creating a toner image, PA0 ii) transferring said toner image to an image receiving element according to this invention, and PA0 iii) fixing said toner image on said image receiving layer, thus creating an image on a background, said image and said background having a different affinity for lithographic printing ink. PA0 i) creating a flow of toner particles from a means for delivering toner particles to an image receiving element according to any of claims 1 to 6, PA0 ii) forming a toner image by image-wise depositing toner particles to said image receiving element through operation of a printhead structure that is coupled to means for image-wise modulating said flow of toner particles and that is interposed between said means for delivering toner particles and said image receiving element, and PA0 iii) fixing said toner image on said image receiving layer, thus creating an image on a background, said image and said background having a different affinity for lithographic printing ink.
The copy sheet with the fused oleophilic image portions is then converted to a lithographic master by treatment with a conversion solution. The conversion step treats the photoconductive coating so that water receptive background areas are obtained. The ink receptive portions are the fused oleophilic toner images.
In second type of printing plate the toner image resulting from step (iii) is transferred from the photoconductive layer to a toner receiving plate on which the toner transfer image is then fixed. In this system the photoconductor can be reused after cleaning. The toner receiving plate does not need a photoconductive coating; any conventional lithographic coating will suffice. Depending on the coating used subsequent chemical treatment may be necessary to render the background areas water receptive.
A third method for electrostatographically producing printing plates is a Direct Electrostatic Printing (DEP) method, wherein electrostatic printing is performed directly from a toner delivery means on a receiving substrate, the latter not bearing any imagewise latent electrostatic image, by means of an electronically addressable printhead structure. After image-wise depositing and fixing of toner on said receiving substrate, it will serve as lithographic printing plate.
Several image receiving layers, applied on a substrate, for producing lithographic printing plates by non-impact printing have been disclosed.
An example of a toner receiving plate provided with a lithographic coating consisting of polyvinyl alcohol, tetraethyl orthosilicate, titanium dioxide and wetting agents is described in U.S. Pat. No. 3,971,660. However, printing plates obtained from these toner receiving plates applying conventional electrophotographic techniques do not yield the desired quality and resolution that can be obtained, for example, with lithographic printing plates, produced by wet methods as e.g. silver salt diffusion transfer processes.
In EP-A 405 016, a method for electrostatographically producing lithographic printing plates has been disclosed wherein dry toner particles of which more than 90% by volume have a particle size diameter less than 10 .mu.m and more than 50% by volume have a particle size diameter less than 7 .mu.m, are used and deposited on a receiving element comprising a plastic film support that is thermostable to a temperature of at least 140.degree. C. and a cross-linked hydrophilic layer thereon, said layer containing infrared absorbing substances in such an amount that the reflection density of said layer in the visible spectrum is between 0.4 and 1.4. This method is said to produce lithographic printing plates with excellent lithographic properties that are dimensionally stable, that do not tear easily and that are capable of duplicating runs in the range of several tens of thousands of copies with good screen reproduction and substantially no fog or scumming.
In U.S. Pat. No. 5,123,920, a toner receiving plate comprising a thermoplastic film support and a cross-linked hydrophilic layer thereon, characterised in that the cross-linked hydrophilic layer of the toner receiving plate either carries on top thereof or incorporates spacing particles forming protuberances on said layer, said spacing particles having an average particle diameter at least twice the average particle diameter of the toner particles. Such a toner receiving element is said to have the advantage over the prior art receiving element that the transfer efficiency of fine toner particles is substantially increased. The use of fine toner particles being required for obtaining high resolution images.
In U.S. Pat. No. 4,686,138 a toner receiving layer for the electrostatographic production of a lithographic printing plate is disclosed, which comprises a water resistant support and an image receiving layer provided thereon which comprises a mixed pigment comprising a synthetic silica fine powder of 20 .mu.m or less in particle diameter and a colloidal silica of 50 nm or less in particle diameter and a binder which is preferably polyvinyl alcohol. The resulting printing plate is said to have good fixability for printing inks and for toners copiers.
In JP-A 61/205190, an image receiving layer is disclosed with a surface roughness of 5 .mu.m to 20 .mu.m (Rmax under JIS-B-0601). It is said that the number of toner particles adhering to the non-picture portion of the receiving material can be reduced, thus reducing the background fog when the lithographic printing plate is used.
In EP-A 701 179 a sheet or web material with a photoconductive ZnO layer for the production of lithographic printing plates by electrophotographical means is disclosed. The layer comprises two kinds of matting agents. Such a material has the disadvantage that the presence of photoconductive ZnO is necessary for the process to work and that, by that need, the degrees of freedom in optimizing the quality of the lithographic printing plates produced on it are limited.
In EP-A 742 107 and DE-A 23 01 466 image receiving layers for use in electrophotographic processes are disclosed, but these disclosures remain silent on the possibility of using the electrophotographically produced images as lithographic printing plate.
In U.S. Pat. No. 4,420,528 a transfer film with a polysulphone resin and NOT carrying a toner receiving layer has been disclosed for replacing paper in an electrophotographic copier, but this disclosure again remains silent on the possibility of using the electrophotographically produced images as lithographic printing plate.
Although the toner receiving layers according to the art cited above are indeed useful for producing offset (lithographic) printing plates by electrostatographic means, further improvements are still desirable.